A thermal-type flow meter is a sensor to measure a flow rate of flown air on the basis of a current necessary for returning a temperature of a heating wire which is changed by the ambient flowing air. The measured flow rate can be represented by aperiodic signal having a frequency corresponding to the flow rate for example. The periodic signal is input to an open drain circuit (or an open collector circuit; the same shall apply hereinafter) for example, and a frequency of a signal output from the open drain circuit is detected by a frequency measurement circuit, so that the flow rate can be detected.
The open drain circuit turns on/off a switching element to output an output signal of a high/low level. The output signal corresponding to the flow rate represented by the frequency of the periodic signal can be output by turning on/off the switching element according to the frequency of the periodic signal.
The output signal becomes a rectangular wave due to the configuration of the open drain circuit. Therefore, high-frequency noises are generated. As a means for relieving the high-frequency noises, it is considered that a plurality of switching elements are connected in parallel at multiple stages, and shifts timing for switching the switching elements so as to output the step-shape output signal. PTL 1 describes a specific example of the open drain circuit in which the switching timings of the respective switching elements are different.